9

Rigid Surfaces 1089
called Replace B ase Surface. You can click this
button and then click a different surface to ac t as
thebaseoftheoffset.
This capability lets you replace a trimmed surface
with its untrimmed version, or vice versa. To
do so, you need to use the Select Objects dialog
(page 1–78). For example, select the trimmed
surface sub-object and turn on the Keyboard
Shortcuts Override toggle (page 3–872).Clickthe
replacement button, press the
H key , expand the
surface’s tree, and then highlig ht the name of the
untrimmed version.
Rigid Surfaces
To i mpr ove p er for m a n ce , yo u c a n m a ke a ny k i n d
of surface sub-object into a rigid surface. The only
editing allowed on a rigid surface is to tr ansform
it at the Surface sub-object level. You c an’t move
arigidsurface’spointsorCVs,orchangethe
number of points or CVs.
Rigid surfaces reduce t he amount of memory
used by the NURBS mo del. Making surfaces r igid
improves performance, especially for large and
complex models.
When a surface is rigid, you can’t see its points
or CVs when you are at the Point or Surface CV
sub-object levels. If the model has only rigid
surfaces and no point curves, the Point and Surface
CV sub-object levels aren’t available at all.
Tomakearigidsurfaceeditableagain,clickMake
Point, Make Independent, Make Loft, or Convert
Surface.
NUR B S and Modi fi ers
In general, you can apply modifiers to NURBS
modelsasyoudotootherobjects.
Yo u c a n a p p l y Edit Patch (page 1–638) and Edit
Mesh (page 1–634) modifiers to NURBS surface
objects.
Tip: To improve performance while you anim ate
your scene, make the surfaces in your NURBS
model nonrelational surfaces (page 1–1116).
Modifiers treat nonrelational surfaces as if they
were independent CV surfaces: you can animate
the scene more efficiently, and then tu rn relational
modeling back on before you render.
Defor ming NUR B S Obj ects
Deform modifiers such as B end (page 1–560)
and Tw ist (page 1–876) operate on CV and point
sub-objects. They don’t change the NURBS model
into an editable mesh object. This means that
youcanuseadeformmodifier,collapsethestack,
and still have a NURBS object that you can edit
further. However, bec ause t he deform modifiers
directly affect CVs and points ( and not the mesh
approxima tion of the NURBS model), they can
produce unexpected results. For example, a Ripple
(page 1–783) modifier does not ripple the surface
if the CVs are farther apart than the wavelength
of the ripples. If you want the modifier to affect
the mesh approximation instead of the CV s, you
can apply a Mesh Select (page 1–719) modifier
first. Then when you collapse the stack, you get an
editable mesh, not a NURBS object.
These are the deform modifiers that collapse to
NURBS:
• Modifiers in the Parametric Modifiers set,
except for Lattice (which collapses to an
editable mesh) and Slice (which collapses to an
editable poly or an editable mesh).
• Modifiers in t he Animation Modifiers set,
except for the world space modifier versions of
PatchDeform, PathDefor m, and SurfDefor m
(which don’t collapse).
Tip: While they w ill collapse to a NURBS object,
the Morpher and Skin modifiers are meant to